Reduction/oxidation, i.e., redox, emulsion polymerizations, to form, for example, emulsified stryrene butadiene rubber, are typically conducted in cool conditions using soaps, and typically terminated by a shortstopping agent. The purpose of the short stopping agent is to make the initiator system inactive under the prevailing conditions, and thereby prevent crosslinking and/or gel formation. Processes for free radical initiated emulsion polymerization terminated by a shortstopping agent are well-known and described in great detail in the prior art. For example, WO 02/38617, U.S. Pat. Nos. 3,402,138; 3,296,177; 5,384,372; and Ullmann's Encyclopedia of Industrial Chemistry Bd. 13, 606, describe such processes for forming, for example, ethylene styrene butadiene rubber, and are each incorporated herein by reference.
There are available commercially a wide variety of shortstopping agents to be used in the aforementioned processes. For example, a widely used short stopping agent is, as described in U.S. Pat. No. 5,384,372, compositions of isopropylhydroxylamine and its salts. In U.S. Pat. No. 6,723,255, it is suggested to employ hydrophobic free radical scavengers based on phenol, nitroxide, and hydroxylamine with isopropylhydroxylamine shortstopping agent in emulsion processes of rubber latexes.
Unfortunately, the redox emulsion processes described above suffer from one or more shortcomings. For example, the processes are only capable of resulting in a monomer conversion of about 65 percent before excess crosslinking occurs or gel formation results. Of course, this means that any unreacted monomer must be recovered for recycling. This may be costly and inefficient. Moreover, if one seeks to increase the monomer conversion beyond 65 percent, then often a large amount of gel and crosslinking results, which may cause further problems, like, for example, equipment fouling.
Accordingly, it would be desirable to discover an improved process whereby higher monomer conversion could be obtained. It would further be desirable if such a process produced a product with less gels and crosslinking than conventional processes. If such a process also resulted in less fouling and corrosiveness despite the high monomer conversion, then it would be further beneficial. It would further be beneficial if such a process could reduce or eliminate the need for employing a short stop agent, monomer recycling, butadiene washing, butadiene distillation, styrene cleaning, and/or other cleaning of inert and/or retarding components.
Advantageously, inventive compositions and inventive processes have been discovered that meet many of the aforementioned needs and also have other desirable characteristics.
In one embodiment, the invention relates to a method for increasing the monomer conversion in a redox initiated emulsion polymerization. The method comprises adding a composition comprising an effective amount of a substituted phenol to the polymerization medium.
In another embodiment, the invention relates to a process for making an emulsified rubber. The process comprises (1) reacting styrene and butadiene in the presence of a soap, water, modifying agent, and an activating agent and (2) optionally terminating the reaction with a shortstopping agent. An effective amount of a substituted phenol is added to the reaction mixture of step (1) to form an emulsified styrene-butadiene polymer having a low gel content and high monomer conversion. The substituted phenol is selected from the group consisting of 2-6-di-t-butyl para-cresol; 2,6-bis(1,1-dimethylethyl)-4-methylphenol; octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)proprionate; benzenepropionic acid, 3,5-bis((1,1-dimethylethyl)-4-hydroxyl-C13-C15 alkyl ester ester; IRGANOX 1520 (2,4-Bis[(octyl-thio)methyl]-o-cresol); IRGANOX 1010 (Pentaerythrit-tetrakis[3-(3,5-ditert-butyl-4-hydroxyphenyl)-propionat]); IRGANOX 1076 (Octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate); and mixtures thereof.
In another embodiment, the invention relates to a composition comprising an emulsified rubber. The rubber comprises units derived from styrene and butadiene monomers. The composition is characterized by: (A) a monomer conversion of greater than about 85 weight percent; (B) at least substantially gel-free; and (C) at least substantially free of crosslinking.
In another embodiment, the invention relates to emulsified rubbers and/or compositions made by the aforementioned methods and processes.
In another embodiment, the invention relates to articles made from the compositions.